A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
As lithographic apparatus are used to manufacture devices with ever smaller critical dimension (CD), i.e. the dimension of a feature or features in lithography shrinks, variations will cause undesirable variation in physical properties of the feature, such as the gate width of a transistor. Calibration and qualification of the apparatus become increasingly important to ensure consistent results and high yield.
Many different parameters of a lithographic apparatus must be measured for calibration and/or qualification of an apparatus or part of an apparatus and in many cases a large number of measurements must be taken to get a suitably accurate and precise measurement of a given parameter. An example is the flatness of the substrate holder (also referred to as the pimple plate or exposure chuck). The substrate holder comprises a flat disc of low-CTE material corresponding in size to a substrate and having on both sides a large number of small projections whose extremities should lie in respective common planes. For an exposure the substrate holder rests on the projections of one side on the substrate table and the substrate rests on the projections of the other side. The spaces between substrate and holder and table and holder are evacuated to clamp the substrate in place. If there is any distortion of the substrate holder or deviation in the height of the projections, the substrate will be distorted in turn, leading to focus spots overlay errors and reduced yield. It is therefore necessary to ensure that the substrate holder is sufficiently flat before use. However, because the projections are small and large in number it is difficult to measure directly whether their ends lie in the correct flat plane.
Conventionally, the flatness of a substrate holder has been determined by placing it in a lithographic apparatus, placing a test substrate on top and measuring the height and/or local tilt of the top surface of the substrate at a large number of positions, e.g. 8,000 to 9,000 for a 200 mm substrate. This is then repeated for ten or more test substrates and statistical methods are used to determine from the resulting data the flatness of the substrate holder, separating out the contribution from the unflatness of the test substrates. However, it has been found that this method can reject substrate holders that do in fact meet qualification requirements, which is wasteful as the substrate holders are difficult and hence expensive to manufacture.
Another example of a calibration and/or qualification method using a large number of test measurements, often over several test wafers is the calibration and/or qualification of an apparatus' overall overlay (X,Y) and focus (Z) performance. Since this takes into account all intrinsic and extrinsic factors that may affect performance of the apparatus, the repeatability of the results is poor.